Process for the reduction of hot-rolled strip with very fine structural grain size



United States Patent 3,432,367 PROCESS FOR THE REDUCTION OF HOT-ROLLEDgiIZREIIP WITH VERY FINE STRUCTURAL GRAIN Wilhelm Stich,Wattenscheid-Hontrop, and Adolf Sickbert, Wattenscheid-Eppendorf,Germany, assignors to Heppenstall Company, Pittsburgh, Pa.

No Drawing. Filed Apr. 5, 1965, Ser. No. US. Cl. 148-12 Int. Cl. C21d7/13 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to aprocess for the reduction of hot-rolled strip to produce a stable finegrained steel and particularly a fine grained steel which will notcoarsen on aging.

The problem of producing a stable fine grained steel is an old and wellrecognized problem. It has long been recognized that hot-rolling wouldreduce grain size but that the fine grain so produced was not stable andwould grow on aging. In order to overcome this problem, it has been thepractice to add alloying ingredients which would induce fine grainformation such as aluminum, vanadium, titanium and zirconium. Suchmaterials are, of course, expensive, require additional handling andtend to promote segregation of impurities in the metal.

More recently, it has been proposed in application Ser. No. 445,405, andnow abandoned, owned by the owner of the present application, to treathot-rolled strip with Water under pressure during hot-rolling prior toOne of the last three reductions of the finishing mill; whereby thestrip or band is quickly cooled and descaled by the water underpressure. The cooling of the band, according to this process, should onthe average amount to about 50 to 100 C. This method has a number offavorable effects. It produces a fine grain which is resistant togrowth, and it produces a smooth surface.

The present invention provides a method for producing fine grainedsteels which eliminates the subsequent problem of grain growth and whichproduces the same smooth surface as disclosed in application Ser. No.445,405. This is accomplished with the use of a small amount of coolingmedia. The present process provides the steps of casting a coarse grainvacuum treated steel free from elements which lead to fine grainformation, solidifying said steel, rapidly reducing the cross-section ofthe solidified steel at an elevated temperature and at a pressuresufficient to reduce the coarse grain structure to a desired fine grainstructure and immediately cooling the reduced steel below the graingrowth temperature prior to the inception of any grain growth.

The structure, which results from the treatment of this invention, isvery fine grained; and the fine grain quality remains substantiallyunchanged even after subsequent annealing treatments which would havecaused considerable grain growth in the materials of the prior art. Itis, for example, possible to obtain a fine grain quality in hotrolledband of deep drawing quality with a final thickness of between three toeight minutes and an ASTM grain size of six to eight.

An unalloyed steel with a carbon content of 0.75% was treated accordingto the process outlined above, i.e., vacuum treated and cast to a coarsegrained structure, hotrolled and sprayed with water in the last stand toproduce an AST M grain size range of seven to nine. This size remainedunchanged after an anneal of one hour at 680 C. in air.

The surface of the hot-rolled band or strip is so smooth that it may beconsidered mirror-smooth for practical purposes. This is attributed tothe fact that the surface roughness previously consisting of varyingscale particles can be completely smoothed out by the rolling operationof this invention.

We have found that an original coarse grain or alternatively a largegrain growth possibility, is necessary if a fine grain structure is tobe obtained in the final rolled sheet by hot-rolling as described hereinand as distinguished from those methods of producing fine grained steelsby metallurgical additions in the melt.

The coarse grain in the slab or ingot must also be induced as a resultof vacuum treatment by means of any of the well-known vacuum methodssuch as pouring stream degassing. This resulting vacuum induced coarsegrain must be reduced under the high roll pressures of modern stripmills and finally quickly cooled. If the final quick cooling step is nottaken, the final strip would possibly grow to a coarse grain evencoarser than the starting grain size. For this reason, it is essentialthat the strip be sprayed with coolant while in the rolling mill. Theresulting eifect of the spraying with coolant is greater, the nearer itoccurs to the end of the rolling process. It is, therefore, essential inthis invention that the steel, which as a result of the vacuum treatmenthas been induced to form a coarse grain, be reduced by high rollingpressures during the rolling of the strip and treated with cooling waterunder pressure during the rolling of the strip and prior to theinception of any grain growth. It is, therefore, particularly effectiveif the steel band receives the impact of the high pressure cooling waterin at least one of the last three reductions in the rolling mill.

In the practice of this invention, the addition of those elements, i.e.,aluminum, vanadium, titanium, and zirconium, which are usually added toinduce a fine grain condition in steel, should be avoided or limited toamounts which are not sutficient to affect the grain growth of the steelfollowing the vacuum treatment.

While we haveillustrated and described certain pre ferred practices ofour invention in the foregoing specification, it will be understood thatthis invention may be otherwise practiced within the scope of thefollowing claims.

We claim:

1. Process for the production of stable fine grained steel comprisingthe steps of casting a coarse grain vacuum treated steel free fromelements which lead to fine grain formation, solidifying said steel,rapidly reducing the cross-section of the solidified steel at anelevated temperature and at a pressure sufficient to reduce the coarsegrain structure to a desired fine grain structure, and immediatelycooling the reduced steel below the grain growth temperature prior tothe inception of any grain growth.

2. Process for the production of hot-rolled steel band of stable veryfine grain characteristics in the finished steel comprising the steps ofcasting a coarse grain vacuum treated steel free from additions ofelements which lead to fine grain formation, solidifying said steel,hot-rolling the solidified steel at normal hot-rolling temperatures in acontinuous strip mill having in excess of three reductions at a pressuresufficient to reduce the coarse grain structure to a desired fine grainstructure, and immediately cooling the reduced steel by spraying withcoolant in any of the last three reductions of said continuous stripmill.

3. A process as claimed in claim 2 wherein the coolant is high pressurewater.

4 References Cited UNITED STATES PATENTS 3,201,288 8/1965 Grange148-12.4 3,250,648 5/1966 Grange et a1. 14812.4 3,245,844 4/1966 Weber148 FOREIGN PATENTS 538,783 3/1957 Canada. 773,071 4/ 1957 GreatBritain.

10 L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

U.S. C1. X.R.

